Ball grinding ceramic wheel containing manganese dioxide

ABSTRACT

Mixtures of abrasive grains and bond compositions within a given range are shown for making ceramic grinding wheels containing predominantly alumina abrasive grits in a fired clay bond having filler and fluxing additives adapted to produce a predominantly crystalline ceramic bond for finish grinding spherical steel balls for bearings, wherein the mesh size and quantity of the abrasive grits and the relative proportions of the clay bonding ingredients including crystalline fillers and fluxing agents, and degree of pressure and conditions of the firing cycle can be varied to predictably modify the grinding characteristics of the resultant fired ceramic wheels in terms of a harder or softer grinding wheel action.

United States Patent 1 Jones [4 Oct. 21, 1975 Primary Examiner-D0nald J. Arnold Attorney, Agent, or FirmRufus M. Franklin Mixtures of abrasive grains and bond compositions within a given range are shown for making ceramic grinding wheels containing predominantly alumina abrasive grits in a fired clay bond having filler and fluxing additives adapted to produce a predominantly crystalline ceramic bond for finish grinding spherical steel balls for bearings, wherein the mesh size and quantity of the abrasive grits and the relative proportions of the clay bonding ingredients including crystalline fillers and fluxing agents, and degree of pressure and conditions of the firing cycle can be varied to predictably modify the grinding characteristics of the resultant fired ceramic wheels in terms of a harder or ABSTRACT softer grinding wheel action.

2 Claims, 3 Drawing Figures 240 GRIT 100 GRIT [75] Inventor: Cecil M. Jones, Worcester, Mass. [73] Assignee: Norton Company, Worcester, Mass. [22] Filed: Jan. 15, 1974 [21] Appl. No.: 433,507

[52] US. Cl. 51/308; 51/307; 51/309 [51] Int. Cl. B24D 3/16 [58] Field of Search 51/307, 308, 309

[56] References Cited UNITED STATES PATENTS 1,364,849 l/1921 Anderson 51/308 1,546,115 7/1925 Beecher 51/308 2,259,468 10/1941 Houchins 51/308 2,475,565 7/1949 Houchins 5l/308 2,544,060 3/1951 Amberg et al. 51/308 i 180 G I m 160 I 1 Lu N g 140 3 a n 2 120 Q 0 Q 3 l l O O O o N K) "/0 ABRASIVE ADDITION BALL GRINDING CERAMIC WHEEL CONTAINING MANGANESE DIOXIDE BACKGROUND OF THE INVENTION In the manufacture of known ceramic grinding wheels, various proportions and types of clays and abrasive grits have been used for a number of years. My invention is concerned with a particular bond composition and procedure for making grinding wheels adapted to be used for the finish grinding of steel spheres for ball bearings. When following the conventional ball grinding procedure a very dense hard wheel having a vitrified bond is used. The steel spheres to be ground, flow around a work holder plate that has grooves in its surface for guiding the balls while the grinding wheel is driven over the table or work holder to engage the balls against a flat side face of the wheel. The ball grinding wheels now used for this activity are bonded with a vitrified or glassy type bond that inherently has a degree of porosity that is reduced as much as possible during the firing procedure by increasing the glassy bond component in the wheel mix, requiring truing of the wheel after firing to condition it for ball bearing grinding.

Traditionally a grinding wheel is made up of an abrasive, a bond to hold the abrasive grits together and voids or pores. Normally a grinding wheel will contain between 40 and 60% volume percent abrasive. Enough bond is added in order to make the wheel as hard as is desired for the particular grinding operation. The more bond or glassy material added to the grinding wheel mix the harder the wheel will tend to be after firing. In the manufacture of ball bearings a very hard and wear resistant grinding wheel is desired. This hardness is achieved by increasing the glassy bond phase in a wheel at the expense of the porosity so that the normal vitrified grinding wheel for ball grinding includes about half abrasive and half bond with the porosity being in the range of less than 5%. In the firing process when there is a 50% weight of bond in the mix, the grinding wheel shape shrinks during conventional firing as much as linear percent. Also, when such a large volume of bond material is present, during firing of the wheel, in order to achieve a proper bond with the grits, the bond component must be fired at a high temperature to make the bond somewhat less viscous. When the relatively large mass of bond ingredients reaches this somewhat more fluid stage, the geometric integrity of the cold pressed grinding wheel shape is not fully maintained so that the ultimate shape of the fired wheel tends to have rounded corners, shows some warpage and shows evidence of slumping or deformation to fit whatever surface it is sitting upon. In addition, invariably some kiln sand or pieces or refractory composition from the firing batts will stick to the bottom of at least some of the conventional ball grinding wheels.

Because of these occurrences, in order to produce a ball grinding wheel to a given size specification with flat surfaces and having the dimensional tolerances required by the ball bearing manufacturer, the conventional glassy or vitrified bonded wheel must be trued to shape after being fired, by grinding off all of the excess material that must be added in order to compensate for the 10% shrinkage, warping, etc. Keeping in mind that such a wheel is as dense and hard a ceramic wheel as can possibly be made, it is seen that this truing or grinding or cutting operation that must be done on the conventional \7iti'ified or glassy bonded wh el, is a very difficult time-consuming and expensive procedure. Because of the natural variability of the raw materials used for glassy bonds, the degree of shrinkage cannot always be reliably predicted. Thus occasionally the wheels will be undersize or oversize so that either rejections or excessive truing is required. The variability in the shrinkage encountered also affects the grinding action and it has been noted for example that if the expected shrinkage does not occur during firing, the wheel will act soft and be unsatisfactory for the custom ers use.

PRIOR ART An example of a vitrified bond for a grinding wheel containing an alumina abrasive grit is disclosed in the U.S. Pat. No. to Milligan 1,910,031, May 23, 1933.

Another example of a vitrified bond for abrasive wheels is shown in U.S. Pat. No. 2,475,565 to Houchins, July 5, 1949. This patent suggests a bond for diamond abrasive grits which particular structure is not especially pertinent with respect to a ball bearing grinding process but the patent may be of possible interest here in showing the use of b entonite and ball clay to gether in a bond for a grinding wheel.

Other patents that illustrate the state of the prior art connected with the use of alumina containing articles, include the U.S. Pat. Nos. 1,572,730 to Locke et al, Feb. 9, 1926; 2,290,107 to Luks, July 14, 1942; and 2,360,841 to Baumann, Jr. et al, Oct. 24, 1944. These patents all show various aspects of the use of manganese oxide as a flux or sintering aid for a ceramic bond in the presence of alumina.

In the U.S. Pat. No. 3,089,764 to Smith-Gorman, May 14, 1963, an abrasive composition useful for barrel finishing is shown. This composition includes an alumina and silica bond composition for an alumina grit in the size range of from 120 to 220 mesh.

French Pat. No. 2,151,509 shows a ball bearing grinding wheel with a ceramic bond having beta alumina and finely ground silicon carbide present to fill the pores.

Although not related to the grinding wheel art, it should be noted that the assignee of the present invention has in the past sold a barrel finishing abrasive product having a ceramic bond composition for alumina abrasive grits, which composition was somewhat like the ceramic bond for the alumina containing grinding wheel forming the subject of this invention. This barrel finishing abrasive composition included: ball clay, bentonite, crystalline fillers, manganese dioxide and about 10% by weight of mesh alumina abrasive grits.

SUMMARY OF THE INVENTION The present invention resides in making use of the compositions here shown for the fabrication of grinding wheels that have particular utility for the finish grinding of steel ball bearings. It has been found that a given cyrstalline ceramic bond can be formulated which together with variations in the relative weight percent additions of a known abrasive grain to a given weight of bond composition, can be used to produce a range of grinding wheel mixes that may be used within certain degrees of pressing during the cold pressing step, for making ceramic bonded wheels in which the grinding action of the completed wheels can be designed to be predictably harder or softer in accordance with predetermined grinding specifications desired for a particular steel ball grinding action. It has been found that the grinding action of a wheel made for this purpose can be predicted from a knowledge of its porosity, i.e. molded density and modulus of elasticity. I have found how to control the porosity and vary the modulus of elasticity in such a wheel to obtain the grinding action specified, by varying the relative weight percent addition of the abrasive grain to a particular weight of bond composition that contains crystalline fillers. I can further adjust the porosity in order to produce a specified modulus of elasticity at a given abrasive content by varying molding pressure. The microscopic porosity inherently present upon firing my bond and abrasive mix, has been found to be directly related to the modulus of elasticity in the relationship that as the weight percent of abrasive grits included goes up relative to the weight percent of bond present in the wheel mix, the modulus of elasticity is reduced and the fired density is less. As the volume of pores in the mass goes up the grinding action of the wheel becomes softer. The converse is true when the relative weight percent of grits to weight of bond is reversed. I can therefore specify a relative weight percentage addition of a given abrasive grit to the weight of the particular bond composition of this disclosure whereby to produce a grinding wheel having a given porosity, modulus of elasticity and predictable grinding action in a wheel that normally may be used as fired, requiring little or no truing as a result of the firing step.

Not only can the grinding characteristics of the finished ball grinding wheel be predicted when making use of my invention, it will be found that the bond matures during firing without becoming so fluid as to permit slumping, warping or shrinking. When firing of the wheel can be accomplished without encountering such undesired behavior, the green or cold pressed wheel shape is retained during firing so that the need for truing can nearly always be dispensed with. Thus substantial savings can be realized over the conventional ball wheel manufacturing procedure.

IN THE DRAWINGS FIG. I is a graph of data compiled to show the relationship of the modulus of elasticity (times that will result upon firing green wheels that have been cold pressed to have the molded density shown;

FIG. 2 is a graph including a further compilation of data showing the relationship of fired density of the grinding wheel, modulus of elasticity and the volume percent of pores in the fired wheels of this invention; and

FIG. 3 shows the variation of the modulus of elasticity in relation to variations in the percent of abrasive content relative to the weight of bond and the effect of different grit sizes of abrasive therein.

DETAILED DESCRIPTION In grinding wheel manufacture it has been established that the greater the modulus of elasticity the harder the grinding action and the more durable the wheel but at the same time its rate of cut is decreased. Conversely, by reducing the modulus of elasticity a faster or softer cutting wheel is produced; however, its durability or life decreases.

The ability to formulate compositions described below, that may be cold pressed to the densities indicated that fall within the enclosed area shown in FIG. 1, enables me to produce a wheel with a specified modulus of elasticity after firing that has the proper durability and relative hardness or softness of cut to satisfy any of a customers steel ball bearing finish grinding requirements. Further, this ability to design variations in durability and speed of cut can be realized by making use of abrasive grains of different grit sizes, as shown in FIG. 3, for example, with U.S. Standard screens fused crushed alumina grits, the modulus decreases somewhat more rapidly with increasing percentages of abrasive content relative to the weight of bond in the wheel as compared with the gentler slope of the curve when increasing percentage additions of 240 grit (corresponding to U.S. Department of Commerce Commercial Standard CS-37l-65) fused crushed alumina grits are added to the given weight of bond in the wheel mix. The same kind of relation between percent abrasive addition and weight of bond exists when silicon carbide in any given grit size is used except that the slope of the curve showing a reduction in the modulus of elasticity is somewhat steeper than the curves show for a correspondingly sized alumina grain. Abrasive grains of alumina oxide are preferred, however, because they are much more compatible with the disclosed ceramic bond composition than are silicon carbide grains which accounts for the different effects of silicon carbide grits and aluminum oxide grits on the modulus of elasticity as stated above.

In FIG. 2 the modulus of elasticity of the wheels of my invention are shown to be variable with respect to both the fired density as well as the porosity, that is, as the modulus represented by the full line curve increases, the fired density increases and the porosity decreases as the modulus increases, as shown by the dotted line in FIG. 2. These three characteristics of modulus of elasticity, density and porosity of the wheel vary in a regular manner. For any particular composition within the range of this invention, the modulus and/or the volume percent porosity can be predicted from the density of the cold pressed green wheel, as shown in FIG. 1. The percent addition of the abrasive grit relative to the weight of bond of my invention within the conventional range of cold pressing pressures, determines the fired density so that a faster cutting softer wheel or slower harder cutting wheel can be produced as needed.

In order to control the properties of these steel ball bearing finishing wheels for any given molding pressure, there are four ingredients that can be varied in the wheel mix to control the properties in the fired product, namely: (1) the clay of the bond, with a bentonite additive; (2) a flux of manganese dioxide and soda or soda ash; (3) a fine grit alumina crystalline additive; and (4) the abrasive grit.

In formulating the bond for my wheels, I preferably start with a conventional vitrifiable bonding clay that notonly supplies alumina, silica and various low melting oxides in the fired bond that assist in the bonding of the abrasive grits but one that also gives some moldability during green wheel processing, that is, the clay compacts well and gives good green strength during the pressing operation. The type of clay is not critical but a conventional clay for bonding, a so-called ball clay, should be selected. For convenience and economics, Mississippi ball clay has been found to be entirely satisfactory.

To formulate a wheel, I start with an amount of clay equal to about 20% by weight of the final mixture and preferably add about 1% bentonite in order to give better compaction properties to the final mix that may be either cold or, in some cases, hot pressed, to form a wheel. The bentonite incidentally functions to lower the melting point of the clay bond mix, but the bond composition can be made without bentonite. When bentonite is used, it is added in percentages no greater than 4 or 5% of the weight of the final mixture simply because it is a clay that shrinks excessively upon firing. The ball clay content of the completed mix can be varied within the range of to by weight of the final mixture. Less than 10% produces a mix that has poor compacting qualities and low bond content while with additions above 30% some of the alumina fillers added to the bond as describedbelow, cannot be effective, so that the ultimate quality of the fired product would not be the highest. J: j,

The second basic ingredient included in the mix is a manganese compound, preferably added as manganese dioxide, which upon firing, acts as a suitable fluxing agent for the clay. This flux material controls the densification of the product upon firing and the degree of density produced is related to the amount of flux put into the mix and the firing temperature as compared to the firing of the same mix with no manganese. For example, when no manganese is present, a hard dense wheel would be produced only if fired at a temperature in the order of 1600C. or 1650C. With a 5% manganese dioxide addition, the firing temperature can be lowered significantly. I prefer to use a 7% manganese dioxide addition because that is the amount that fires most satisfactorily in a standard Orton cone l2 kiln. When firing at cone 12, a manganese dioxide content of less than 5% would result in a soft product, but at higher temperatures beyond a preferred range, the mixture could be fired to reach the optimum wheel density. If the manganese content of the mix is increased about 9% and the product is fired to cone l2, excessive bloating, blistering and slumping of the product will result. For firing temperatures within the conventional ceramic wheel firing range of from Orton cone 10 to cone l6, manganese dioxide is preferably added in the range of from 6 to 8% by weight of the final mix. The manganese dioxide content required, however, is related to the particular firing temperature desired for the product and an increase in manganese is needed to satisfy the lower firing temperatures and, conversely, a lesser amount is needed if higher temperatures are used.

I theorize that when the MnO functions in its role as a ceramic flux, the Al O SiO content of the clay bond becomes fluid at a lower temperature and the resulting viscous glass, through surface tension, then draws the mass of the wheel together to achieve the resulting density and modulus of elasticity. The drawing together or controlled shrinkage of bond around the abrasive particles reduces the pores trapped by the pressing operation achieving the dense product.

Also during firing the MnO loses oxygen to form Mn O and eventually Mn O This free oxygen serves to reduce the residual carbon in the ball clay and causes the glass formers such as SiO to be fully oxidized.

I also separately add soda as an ingredient of the filler material described below. The soda ash in an amount of about 1.5% of the dry weight of the final wheel mix serves as an additional fluxing component to make the bond more fluid. When soda is present in a selected filler ingredient of the bond, a separate addition may not be required provided the ultimate dry mix for the wheel includes approximately 1.5% to 1.6% soda content.

This glassy ceramic bond developed under the influence of the MnO and soda forms a strong adhesion to the A1 0 abrasive particles. It is known that a glue or adhesive must wet a surface in order to stick to the surface and this viscous glass forms at the lower tempera tures under the influence of the flux and has such a degree of surface tension that it draws the abrasive particles together although it does not wet the abrasive sur faces in the normal manner of a glue. The viscous glassy bond is in fact also a powerful solvent for the A1 0 abrasive so that the surface of the abrasive is dissolved by the fluid glass, thus insuring an intimate contact or wetting of the abrasive by the glass.

The manganese oxide can be used in any form of a mineral or chemical that is a combination of MnO- Al O -SiO such as pyrolusite, hau smannite, manganite, rhodochrosite, rhodonite or spessartite. Pyrolusite is preferably used because it is most available and least cost. Soda can be added in the form of sodium carbonate which loses CO to form soda if needed, or provided as a component of one of the other ingredients of the bond mix, as will appear below.

The third basic ingredient of the wheel of this invention, is crystalline alumina in the form of dust collector fines and/or Bayer process alumina. Such crystalline alumina will have some abrasive characteristics when the fired wheel is in use and it is added to the mix of this invention primarily as part of the bond material. Also, this component of the mix gives the essentially crystalline bond its hard, durable, long-wearing properties as well as supplying some of the abrasive action of aluminum oxide crystals for grinding the balls. It is believed, that this fine grit alumina material that is distributed homogenously throughout the bond, makes this prod uct superior to previous wheels used for grinding ball finishing grinding wheels. Any crystalline form of fine grit aluminum oxide having particles sometimes as large as microns, but mostly in a size range of under 50 microns, can be used for this purpose in the mix with the average particle size in the range of from 7 to 12 microns. The alumina. filler is present in an amount of from about 15 to 60% of the dry weight of the final mix.

Certain fused alumina products have a soda impurity therein and when this material is crushed, the soda which causes beta alumina crystals to form, tends to be selectively separated in the dust collector fines because beta alumina is softer than the alpha alumina form. The softer beta alumina tends to be crushed to a fine size and is collected in the dust collector in the crushing system. When dust collector fines are used as a filler, sufficient soda is present in the filler. If Bayer alumina is used, the soda content must be sufficient to insure a proper addition of the soda content to the mix.

The fourth ingredient to be added to the mix is the primary abrasive grits, which material serves a two-fold function; first, the larger abrasive grits do increase the rate of stock removal in the grinding of steel balls for bearings and secondly, the larger grits of fused crushed aluminum oxide serve in conjunction with the bond mix to control the modulus of elasticity by controlling the degree of porosity resulting from firing the wheel composition of this invention. Control of porosity and modulus, as above stated, makes the wheel either slower or faster cutting and thus the rate of cut and grinding ability of the grinding wheel is directly related to variations of the percent weight addition of the abrasive grit component relative to the weight of bond mix in the Wheel. As the relative weight percentage of abrasive grits to weight of bond increases, so does the porosity. As the volume of the bond mix including the fine alumina dust collector fines and/or Bayer alumina shrinks during firing to form the dense ball grinding wheel, the larger particles of abrasive grits that are already preshrunk, cause the pores to form as they do not shrink when the bond mix does. This differential shrinkage between the preshrunk abrasive grit component and the mix composition that includes the alumina containing ball clay, manganese oxide and soda flux and dust collector and- /or Bayer alumina filler, explains why the abrasive grits act to reduce the modulus of elasticity by reducing the density and increasing the porosity of the ball grinding wheels in a controlled and predictable manner.

A percentage range of larger sized abrasive grit additions is shown clearly in FIGS. 1 and 3. The percent of such alumina abrasive grit relative to a given weight of bond should preferably fall in a range above a 10% addition, up to an addition of as much as 50% fused crushed alumina abrasive grits, which upper limit is determined relative to the total weight of bond mix including the dust collector fines and/or Bayer alumina which are included in the weight of the bond. 1 find that a 50% weight of abrasive grits relative to the total weight of the ingredients to be the top limit for this product, and the other 50% of the weight being the bond including the clay, the manganese and soda, and the alumina dust collector fines and/or Bayer alumina in order to produce a fired wheel that does not have a modulus of elasticity below approximately 110 X l dynes/cm While the percent of relatively larger sized abrasive grits in the wheel can drop to 0 provided the fine grit filler component in the bond is increased to as much as 60% of the weight of the resulting wheel, this is not a preferred mix primarily because of the difficulty in pressing the mixes containing the finer grits to produce the desired density, also because of the higher shrinkage that is encountered on firing the wheel formulated with such a mix, and lastly because of the excessive hardness of the resulting wheel, it grinds very slowly. Such a hard dense wheel will stand up to extremely high pressures and when production machinery is available to fully utilize such a wheel, a composition of this mix should be found to be highly useful.

Differently sized wheels 24 and 36 inches in diameter, 3 and 4 inches thick with a 12 and 18 inches diameter center holes have been made in accordance with my invention for ball grinding, ranging from very hard to very soft with the following formations:

-Continued Hard to Soft Hard to Soft Dry Dry Dry Dry Dry Dry Dry Dry Wt. Wt. Wt. Wt. Wt. Wt. Wt. Wt. 7r K grit size 15 25 31 37 47 El-240 grit 13 31 47 size After the dry mixes were completed 6% by weight of water was added and the mixing was completed.

The green wheels were cold pressed at 2,000 No. per square inch from such formulations.

After drying in air they were fired at Orton Cone 12.

*38A is a very pure form of fused alumina abrasive having a concentration of beta alumina fines therein. these fines are gathered from dust collectors used with certain alumina crushing operations in the plant, and are sold by Norton Company. The dust collector fines have a generally weak shape and vary greatly in size from 2 to 50 microns with some particles as large as microns, the average size of the dust collector fines being about 7 to 12 microns. is a conventional Bayer process alumina. The particle size of this crystalline alumina varies from about 2 to 15 microns with an average size of 7 microns. is Regular" fused alumina grit sold by Norton Company derived from are furnace fused bauxite.

Data typical of the above wheel structures are the following:

% Abrasive having a size Modulus of 100 US. Std. Screen, Density of Volume Fused Crushed Al,O gm/cc of Pores Elasticity* A typical chemical analysis by weight of such a wheel is:

79 A1 0, 13 SiO 6.4% Mn o 1.6% Other oxides Where soda ash is added to replace the soda in 38A D C F, as when Bayer alumina is used as a filler, the typical fired wheel will have the following composition:

Weight Ball clay 20 Bentonite l MnO, 7 Bayer Process A1 0 40.5 Soda ash 1.5 Fused alumina 100-Grit 30 sated by increasing the weight percent of abrasive grit content of the wheel relative to the weight of bond content in order to decrease the modulus and increase the porosity, thus improving the rate of cut.

The compositions of my invention are mixed, and then are preferably cold pressed in a mold within a range of from /2 to 1 ton pressure per square inch, are then shaved and dried, utilizing conventional grinding wheel manufacturing procedures.

Pressures within a range of from 2/10 of a ton per square inch up to 2 tons per square inch can be used. if hot pressing facilities are available pressures within a range of from 500 pounds per square inch and lower should be used to accommodate the strength of the mold. The firing procedure for these compositions is similar to those used for firing conventional ball wheel compositions. In the preferred processing, tunnel kilns are used, the wheels to be fired being set in sand on a suitable batt to be fed through the kiln in a normal procedure at about Orton Cone 12. While the wheels described herein do shrink from 5 to 9% lineally, the geometric integrity of the wheel is maintained and there is no obvious melting or slumping, bubbling or frothing or any evidence whatever that the bond melts or liquifies during the firing process. It should be noted, in fact, that all blemishes, chips, marks, scratches and the like that are on the surface of these wheels in the unfired state will be clearly evident in the wheels after they are fired.

After firing, the wheels retain their cylindrical shape and the surfaces are flat so that they can be fitted, as fired, into the customers chucks to be used in the above-described known ball bearing grinding process without the lengthy and expensive truing of the wheels formerly made for the steel ball grinding process. This invention not only eliminates most of the costs incident to the truing operations required in the prior art, but it also reduces processing time so that the wheels can be delivered to the customer in accordance with his specifications in much shorter time periods. After the firing cycle is complete, the wheels of my invention are inspected for cracks, dimensions, modulus of elasticity and density. Most wheels made by my procedure pass inspection and are normally ready for shipment to the customer without requiring any dressing or further preparation.

A range of different compositions within the concept of this invention are needed in order to satisfy customers needs and such wheels having a modulus of elasticity of from 110 to a modulus of between about 170 to 180 (all X 10 dyneslcm may be designed by reference to FIG. 1, using both 240 abrasive grit and 100 abrasive grit made from fused crushed aluminum oxide. Compositions within the modulus and density range represented within the closed area defined by the 10 to 50% abrasive lines and the curved lines A and B in FIG. 1, will have a tolerable linear shrinkage as described above that essentially eliminates the necessity for truing the fired wheel. If the wheels are not pressed at least to a minimum degree, the green wheel cannot be properly handled. The line A represents the modulus and porosity (density) resulting from firing wheels of the indicated composition press-ed to the 2/10 ton per square inch pressure. The line B represents the results attained with a pressure of 2 tons per square inch. Too high a pressure causes laminations to form and I therefore have indicated the enclosed area of FIG. 1 to be my preferred teaching.

It is to be expected that in some instances a light truing cut may be needed to complete the manufacture of a fired wheel, but normally there will be very little or no truing involved and most of these wheels can be used as tired. Special shapes and recesses can be produced in the pressed but unfired wheel by shaving and since no truing is normally required, it is apparent that specially shaped wheels can be made much more cheaply. The use of an abrasive grit addition in sizes ranging from 240, or 150, or 120, or 100, or grit size will affect only the relationship of the weight percent abrasive addition to the weight of bond to control the modulus of elasticity and control of the amount of linear shrinkage encountered during firing.

Silicon carbide abrasive grits can also be used within the range of from 4 to 12% by weight. This compares with the range of from 15 to 47% by weight for the alumina abrasive grit additive. The use of silicon carbide does not form a part of the preferred teaching, however, because firing difficulties affecting the final appearance of the wheel have been encountered requiring more truing of the finished silicon carbide containing wheel as compared with the wheels made with alumina grits in accordance with this teaching.

What is claimed is:

l. A hard dense ceramic wheel for the finish grinding of spheres for ball bearings, the wheel having a composition comprising a clay component in the ceramic bond in the range of 10% to 30% by weight of the dry mix from which the wheel is made, a crystalline alpha alumina filter present in said. bond in an amount of at least 15% by weight of the dry mix, particles of said filler being less than about 44 microns maximum size in their longest dimension; abrasive grits in said bond being coarser than said filler and in a size range of 80- grit size and finer based on the U.S. Standard screens; said abrasive grits being selected from the group consisting of A1 0 SiC and mixtures thereof; said grits being present in a range of from 10 to 50% by weight of the dry mix; and said bond and abrasive mix including at least 5 to 9% by weight of manganese oxide, said wheel having a modulus of elasticity of from X 10" dynes/cm to X 10 dyneslcm and a density of from 2.8 to 3.25 grams/cm? 2. A wheel in accordance with claim 1 wherein said alpha alumina filler is selected from the group consisting of alumina dust collector fines and Bayer alumina. 

1. A HARD DENSE CRERAMIC WHEEL FOR THE GRINDING OF SPHERES FOR BALL BEARINGS, THE WHEEL HAVING A COMPOSITION COMPRISING A CLAY COMPONENT IN THE CERAMIC BOND IN THE RANGE OF 10% TO 30% BY WEIGHT OF THE DRY MIX FROM WHICH THE WHEEL IS MADE, A CRYSTALLINE ALPHA ALUMINUM FILTER PRESENT IN SAID BOND IN AN AMOUNT OF AT LEAST 15% BY WEIGHT OF THE DRY MIX, PARTICLES OF SAID FILLER BEING LESS THAN ABOUT 44 MICRONS MAXIMUM SIZE IN THEIR LONGEST DIMENSION, ABRASIVE GRITS IN SAID BOND BEING COARSER THAN SAID FILLER AND IN SIZE RANGE OF 80-GRIT SIZE AND FINER BASED ON THE U.S. STANDARD SCREENS, SAID ABRASIVE GRITS BEING SELECTED FROM THE GROUP CONSISTING OF A12O3, SIC AND MIXTURES THEREOF, SAID GRITS BEING PRESENT IN A RANGE OF FROM 10 TO 50% BY WEIGHT OF THE DRY MIX, AND SAID BOND AND ABRASIVE MIX INCLUDING AT LEAST 5 TO 9% BY WEIGHT OF MANGANESE OXIDE, SAID WHEEL HAVING A MOLDULUS OF ELASTICITY OF FROM 110 X 10**10 DYNES/CM2 TO 180 X 10**10 DYNES/CM2, AND A DENSITY OF FROM 2.8 TO 3.25 GRAMS/CM.
 2. A wheel in accordance with claim 1 wherein said alpha alumina filler is selected from the group consisting of alumina dust collector fines and Bayer alumina. 